Good question! Here are the 3 most common reasons:

Petroleum mineral oil starts out as a fluid that is pumped from the ground as crude oil, is later refined and then strengthened with an additive package. The result is a lubricant that is relatively inexpensive to produce but must be changed frequently because it consists of a mixture of chemically "weak" molecules.

A synthetic lubricant on the other hand, is built "from the ground up" with strong, uniform chemical components. The synthetic product not only lubricates better but lasts much longer. We would expect this process to be somewhat more expensive, and it is.

On the other hand, it is equally clear that an oil which improves economy saves money. Also, in the long run, an oil that reduces wear saves money too. So does an oil which is very stable and can therefore be used continuously over much longer periods of time. (What is commonly known as "extended drain intervals") In the case of Amsoil synthetics, ALL of these benefits are present at the same time. So although the initial price may be higher, the actual "cost of use" is typically lower than that of a mineral oil. Using the product is also much more convenient, because typically (depending on product used and the annual mileage on the vehicle), only one oil change is required per vehicle per year.

I have personally tracked actual engine oil expenses for more than 20 years now, and can readily demonstrate that I save around $200 (U.S.) per passenger car, per year, using Amsoil lubricants. Our family currently owns three vehicles, which translates into annual savings of $600 versus using a less expensive petroleum oil - which really isn't so cheap after all. Yes, it's easy to fixate on initial price, because we have all been brain-washed by careful advertising to ignore the reality of "total cost of use". When it comes to expensive automobiles, that's a dangerous trap. The right premium oil is the best maintenance "insurance policy" you can buy.

As I have tracked oil change expenses over the years, an interesting shift has occurred. The ratio between the cost of oil and the cost of labor (to change the oil) has increased dramatically. In the U.S. anyway, cost of labor has sky rocketed, while the cost of oil (over the same period of time) has stayed fairly stable. [Not including the recent international upswing in the cost of crude oil]

The obvious conclusion is this:  Anything we do to reduce the labor (for oil changes) is going to leave more money in the bank. On my vehicles I make only one oil change per year based on Amsoil service guidelines. Using the average annual mileage of my commuter car, I avoid the labor associated with 5 oil changes - based on recommended petroleum oil change intervals. At the current cost of labor (about $20 U.S. per oil change), that equates to $100 in labor I avoid each year - on that single vehicle. Furthermore, that's 5 times a year that I don't have to drive over to Larry's Lucky Lube and wait in line for them to change oil!